This invention relates to a process for forming color images utilizing color intensification. More particularly, it relates to a process for forming high density color images by imagewise exposing a photosensitive material comprising a low content of silver halide and developing the material wherein redox reaction between a reducing agent and an intensifier is catalyzed by the developed silver.
Color image forming process of the multi-layer structure subtractive color mixing type utilizing color development is one of the most widely used color image forming processes. A number of attempts for saving the amount of silver used in photographic material have been made from various aspects, including use of 2-equivalent couplers, use of auxiliary agents capable of improving covering power application of toning technique, bleaching redevelopment, selection of a color material having a high coefficient of absorption, and the like.
In the image forming reaction, silver halide is a phototsensitive substance and at the same time, an oxidizing agent participating in dye formation. It has been proposed to save the amount of silver by substituting another compound for that fraction of the silver halide serving for the oxidizing function. It is known as a color intensified image forming process.
Oxidation of a paraphenylenediamine color developing agent by decomposition of hydrogen peroxide on catalyst surface in the presence of a coupler and subsequent formation of a dye by coupling with the coupler (this development is known as color intensification) is described in Friedman, "History of Color Photography", 2nd Ed. (1956), page 406. Other various photographic processes utilizing decomposition of peroxides on noble metal surface are described, inter alia, in West German patent application (OLS) Nos. 1,813,920, 1,950,102, 1,961,029, 2,044,833, 2,044,993, 2,056,360, 2,056,359, and 2,120,091.
Color intensification by cobalt complex salts on noble metal surface is described, inter alia, in Japanese patent application Kokai Nos. 48-9728, 48-9729, 48-48130, 49-84229, 49-84240, 49-97614, 49-102340, and 49-102314.
Color intensification by halogenous acids such as chlorous acid is described, inter alia, in Japanese patent application Kokai Nos. 51-53826 and 52-13335. Color intensification by iodoso compounds such as iodosobenzoic acid is described in Japanese patent application Kokai No. 52-73731.
These peroxides, halogenous acids, iodoso compounds, and cobalt (III) complexes are a class of compounds having intensifying ability which are known as intensifiers. Processing solutions containing such intensifiers are known as intensifying solutions or baths.
Among others, color image enhancing techniques utilizing the catalysis of peroxides and cobalt (III) complexes are most typical, and hydrogen peroxide intensification is believed best from the standpoint of amplification efficiency. These processes intensify images by imagewise exposing a silver halide photosensitive material, developing the material, the resultant developed silver serving as a catalyst, and efficiently producing an oxidized developing agent on the catalytic silver nuclei through redox reaction between an intensifier and a color developing agent, thereby forming a fully dense dye image. Thus, the intensifying treatment is generally a treating process involving immersing in an intensifying bath a color photosensitive material which has undergone color development or black-and-white development and then immersion in a color developing solution, whereby a latent image is first grown by development, the photosensitive material film coating is then caused to retain the color developing agent, and the color developing agent entrained with the photosensitive material is used to intensify images in the intensifying bath through redox reaction between the intensifier and the color developing agent on the developed silver nuclei.
To effectively carry out image intensification, it is necessary that a relatively large quantity of the developing agent is retained in the photosensitive material coating and hence, entrained into the intensifying bath and that the developing agent in the coating is rapidly oxidized on the catalytic silver nuclei by the intensifier before the developing agent diffuses away during intensification. One approach from this aspect is to use a highly oleophilic developing agent in a developing solution as disclosed in U.S. Pat. No. 3,816,134 and Japanese patent application Kokai No. 53-30333.
These intensifying baths are generally at an alkaline pH level, and it is desired to reduce their pH as low as possible for handling safety.
The image intensifying treatment has the disadvantage of adversely affecting simple processing because the intensifying bath immersing step adds one step to the conventional process. From the standpoint of simplifying intensification, U.S. Pat. No. 3,847,619 and 3,923,511 describe a technique of concurrently carrying out development, intensification and bleaching in a single bath using a cobalt (III) complex.
The possibility of simultaneously carrying out development and intensification in a single bath (mono bath) using hydrogen peroxide known to have a high amplification efficiency is indicated in West German Pat. No. 1,813,920 and Japanese patent application Kokai Nos. 52-13335 and 55-127555. However, little single bath developing/intensifying effect can be achieved in low silver content photosensitive material simply by adding hydrogen peroxide to a conventional color developing solution because of poisoning by potassium bromide present in the developing solution, usually in an amount of several hundred milligrams per liter.
One prior art method addressing this problem is a color intensified image forming process which can form satisfactory color images with less fog even in extremely low silver content color photosensitive materials at a high amplification factor, using a minimal number of processing baths as disclosed in Japanese patent application Kokai Nos. 58-18629, 58-127926, 58-137837, 58-140741, and 59-65843.
These image forming processes utilizing intensifiers have the advantage of saving the quantity of silver, but suffer from the disadvantage that the intensifiers used are unstable in aqueous solutions so that the intensifying solutions lose their activity in a very short time. This disadvantage is more severe particularly with peroxides having a high intensifying activity, and also occurs with cobalt (III) complexes having less activity than the peroxides.
In addition, a developing solution having a peroxide or another intensifier copresent with a developing agent, which is referred to as a combined developing/intensifying solution, suffers from a further reduced lifetime.